Minimum thickness for exposed steel not accessible for repainting: For steel members exposed to weather and not accessible for repainting, what should be the minimum steel thickness to ensure durability?

Introduction / Context:
Durability of outdoor steelwork depends on corrosion protection. When periodic repainting is not feasible (e.g., inaccessible locations), the member's minimum thickness should be sufficient to tolerate expected corrosion losses over the design life, complemented by protective systems if any.

Given Data / Assumptions:

• Member is exposed to weather.
• Member is not accessible for maintenance repainting.
• Working-stress/design guidance typical of building practice.

Concept / Approach:

Thicker sections provide a corrosion allowance and slow section loss rate in early years. Traditional steelwork practice specifies a higher minimum thickness for unmaintainable, exposed members than for members accessible for repainting. A commonly used minimum is 8 mm in such cases, whereas 6 mm may be permitted when repainting access is available.

Step-by-Step Solution:

Verification / Alternative check:

Handbook recommendations and long-standing practice adopt 8 mm for such conditions to accommodate corrosion, sealing welds, and fabrication tolerances.

Why Other Options Are Wrong:

• 4.5 mm, 6 mm: typically used for protected or maintainable members; too thin for inaccessible, exposed conditions.
• 10 mm: more conservative and heavier than necessary for many building applications.

Common Pitfalls:

• Ignoring connection plates which may govern minimum thickness.
• Overlooking galvanizing/weathering steel options that modify thickness requirements.

Final Answer:

8 mm.